In electrophotographic printing, a toner image which has been transferred onto a receptor support, such as paper or plastic film, must be fixed in order to yield a useable image.
In a method using thermal energy, a toner image formed on a receptor support is melted by heating so as to adhere to the support, and for this purpose the toner image is generally pressed by a roller heated-up to the temperature at which the material composing the toner becomes adhesive. When the receptor support is paper, the toner is tightly fixed to the paper since the toner is melted and part of the toner is absorbed into the fibres of the paper.
A heat and pressure fusing device for fixing a toner image as described, comprises a fixing roller and a pressure roller in contact therewith, the receptor support having a toner image on it being passed between the nip of said rollers to fix the toner.
Inside the fixing roller, there is usually provided a heater which is switched on and off, or which is modulated in response to the output signal of a temperature sensor measuring the surface temperature of the roller, in order to obtain a reasonably constant surface temperature of said roller.
The fixing roller generally is a roller composed of a cylindrical metallic tube of aluminium or the like which is covered with a resilient layer, e.g. a layer of silicone rubber. The resilient layer may be a one-layer construction, but often this resilient layer is composed of a relatively thick inner layer of rubber comprising filler agents increasing the thermal conductivity thereof, and a thin outer layer having releasing characteristics for the receptor support in contact therewith. The fixing roller is rotatably mounted and driven by suitable motor means at a speed corresponding with the speed at which the toner image has been formed on the receptor support, but which may also differ therefrom. The pressure roller may have the same construction as the fixing roller but may also differ therefrom, e.g. by the thickness of its resilient layer, the composition thereof, the amount of its heating, its diameter, etc.
The nip between both rollers, more exactly between the resilient coverings of these rollers, is in fact the area where heat and pressure produce the fusing and thus the fixing of the toner image of a support conveyed between the rollers.
Whereas the fixing of black-and-white single layer toner images does not raise major problems in practice, the fixing of colour images is more difficult since in fact four superimposed toner colour separation images have to be fixed (Y, M, C and K) and this increased amount of toner requires a longer fusing time demanding a nip with a larger length (i.e. the dimension l of the nip measured in the transport direction of the support), unless the fusing speed is proportionally reduced.
A longer nip requires resilient layers on the rollers of increased thickness, but this raises a problem. Silicone rubbers (and similar materials) have a low thermal conductivity and therefore the surface temperature of the fixing roller largely varies with the passing-through of the receptor support because there is not sufficient time available for the heater of the fixing roller to compensate for the temperature drop of the roller surface caused by the contact with the support sheet being at room temperature. Even sophisticated temperature control systems are hardly capable of maintaining the surface temperature of common fixing rollers within acceptable limits. As a result image characteristics such as colouring, density and gloss vary whereby consistent image qualities are difficult to obtain.
Another disadvantage of a fixing roller having a resilient covering with increased thickness in order to obtain a nip with sufficient length l, is the reduction in lifetime of such covering caused by its increased deformation in the nip.